The operation of electric drive systems, especially AC systems such as an AC motor or a generator or a driven unit connected to an AC motor, is often hampered by so-called bearing currents. In motors, the internal voltage division of the common mode voltage over the internal stray capacitances of the motor may cause shaft voltages high enough to create high frequency bearing current pulses. This can happen if the shaft is not earthed via the driven machinery while the motor frame is earthed in the standard way for protection. The bearing current pulses, namely electric discharges taking place in the lubricating oil film between the ball and the races of the bearing, cause erosion of the race. As a result, the bearing may need replacing after only a short time in service. This issue is addressed in for example the document ‘Technical Guide No. 5—Bearing Currents in Modern AC Drive Systems’, ABB Automation Group Ltd., 1999, as well as the document WO2005/034314.
Of course, it is essential to be able to detect and/or measure the bearing currents in order to prevent the bearings of an electric drive system from breaking down and to take measures for avoiding the bearing currents. In the above mentioned document from ABB Automation Group Ltd., it is suggested to measure bearing currents using a specially designed, flexible, air-cored, Rogowski-type current sensor, which sensor is used to measure the current directly at certain measurement points within an AC motor. However, a drawback with this solution is that in order to reach the required measurement points within the AC motor, the AC motor has to be stopped and/or at least partly dismantled, which is cumbersome and time consuming and has to be carried out by experienced and dedicated personnel.